Testing for Crash & Safety Simulation
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Transcript of Testing for Crash & Safety Simulation
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Testing for Crash & Safety Simulation
Hubert Lobo
DatapointLabsNew York, USA
CAHRS Workshop
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DatapointLabs
Research quality material testing
ISO 17025 production environment
Results in 5 days (48 hour RUSH service)
Web-based quotation & data delivery
Domain expertise in CAE material calibration
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expert material testing
TestPaks® = Materials testing + CAE material parameter conversion
metal, plastic, foam, rubber, composites…
over 20 CAE software codes
testing
Your CAE
data conversionmaterials +
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Topics
A test philosophy for representing rate dependency of materials Experimental technique including sampling and specimen geometries Assessment of crash material data quality, expected trends & validation Specific comments for unfilled and fiber-filled polymers, foams, rubber and metals.
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Getting pertinent properties
Importance of measuring the right property
Artifact free dataProperly designed experiments
eg. not using crosshead displacementto calculate strain
Traceable data (ISO 17025)NIST traceable instruments
Certified trained technicians
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Getting the right samples
Spatial variationProperties vary with location
Forming, stretching, molding…
Environmental variationAgeing and conditioning
Process variationDegradation from processing
Recycled materials
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Metals
Relatively well behaved
Models designed to match behavior
Challenges lie with post yield non-Mises failure envelopes
Scaling of yield surface with strain rate
Work of Nakajima, Dubois, Hooputra
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Plastics
Not well behaved
Models not designed for plastics crash simulation
Complex models are expensive
Can we develop best practices for adapting common models to plastics
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Plastics Behavior - Basics
Non-linear elasticity
Elastic limit well below classical yield point
Significant plastic strains prior to yield
Post-yield with necking behavior
0
5
10
15
20
25
30
35
0.0 5.0 10.0 15.0 20.0
Engineering Strain (%)
Engin
eerin
g St
ress
(MPa
)
data
Plastic Point
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Plastics Rate Effects
Modulus may depend on rate
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Plastics Rate Effects
Fail strain may be rate dependent
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Material Testing
Instron servo-hydraulic UTM
Dynamic load cell
Test at 0.01, 0.1, 1. 10, 100/s strain rates
Temperatures: -40 to 150C
tens_slow.mpg
Tens.mpg
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Test Specimens
ASTM D638Type V
PreparationCNC from plaque
CNC from part
Molded
Variabilityprocessing
orientation
thickness
gate region
E1
E2
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Modeling simple ductile plastics
Modulus is not rate dependent
Large strains to failure
Post-yield necking
Plasticity curves vary with strain rate
Failure strain independent of strain rate
LS-DYNA, ANSYS, ABAQUS, PAMCRASH
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Choosing EMOD
0
5
10
15
20
25
30
35
0.0 5.0 10.0 15.0 20.0
Engineering Strain (%)
En
gin
ee
rin
g S
tre
ss (
MP
a)
data
Plastic Point
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Post-yield with necking (Deihl)
0
10
20
30
40
50
60
70
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Strain mm/mm
Str
ess
MP
a
UTM1-39.62-1
UTM1-39.62-2
yield point
necking starts
neck propagation
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Fail Limitations
When FAIL f(strain rate)
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0
5
10
15
20
25
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35
40
1.E-02 1.E-01 1.E+00 1.E+01 1.E+02
Strain Rate(/s)
Ten
sile
Str
engt
h (M
Pa)
data
Cowper Symonds
Eyring
Modeling Rate Dependency
Cowper SymondsDoes not correlate well with plastics rate dependency
LCSRCapture model independent behavior
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Eyring Model
Eyring ModelYield stress v. log strain rate is linear
Best form for plastics
Fit yield stress v. log strain rate data to Eyring equation
Can submit to LSDYNA MAT24 as table using LCSR
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MAT24 validation
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10
20
30
40
50
60
0 0.05 0.1 0.15 0.2 0.25 0.3
Strain (mm/mm)
Str
ess
(MP
a)
LS-DYNA Simulation
Tensile Experiment
MAT 24 Model
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Brittle plastics
Modulus is rate dependent
Small strains to failure
Brittle failure
Failure strain decreases with increasing strain rate
LSDYNA MAT19
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Methodology for MAT 19
Determine elastic limit at quasi-static strain rate
Use elastic limit for von-Mises yield
Define failurefailure stress v. strain rate table
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Ductile-brittle transitions
Non-linear behavior
Failure depends on strain rate
ModelsLS-DYNA MAT89
PAMCRASH 103
Abaqus *ELASTIC *PLASTIC,Rate
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Fiber Filled Plastics
Digimat MXMaterial model reverse engineered from standard experiment
Perform injection-molding simulation
Apply Digimat material model to transfer data to crash simulation
Crash model has spatially oriented properties
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Basic Digimat TestPak Protocol
Mold 100X300X3.16mm plaquesEdge gated on 100 mm end
Long flow length
Fully developed flow
Highly fiber orientation
Cut test specimens by CNC
5 specimens each (0°, 90° )
Obtain true stress-strain data
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Advanced Models
MATSAMP (LS-DYNA)
Standard rate dependent model
Add non-mises failure envelopeCompression
Shear
Add triaxialityPost yield transverse strain
Add unloading
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Pros and Cons
Better failure envelope modeling
Greater cost
More complex model
Greater simulation accuracy in difficult cases
Cost-benefit not certain for general use
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Foams
Different deformation modesCrushable
Elastic with or without damage
Visco-elastic
Large volumetric strain component
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Effect of Poisson’s Ratio = 0
Material compacts by eliminating airNo lateral deformationPoisson’s Ratio -> 0Axial strain volumetric strainTrue for
open cell foamscrushable foams
May not be true forclosed cell foamselastomeric foams
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Typical Stress-Strain Data
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
0 20 40 60 80 100
Engineering Strain (%)
En
gin
ee
rin
g S
tre
ss (
MP
a)
Zone 1 Zone 2 Zone 3
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Test Strategy
Compressive stress-strain5 decades of strain rate
.01, .1, 1, 10, 100 /s
Temperatures -100 to 150C
Optional testsTensile (for cut-off stress)
Shear (as required)
Hisp_comp.mpg
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Test Instruments
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PU Foam-stress strain
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Strain (mm/mm)
Stre
ss (M
Pa)
0.01/s fit
0.1/s fit
1/s fit
10/s fit
100/s fit
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PU Foam- rate effects
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PU Foam recovery
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0 20 40 60 80 100
Engineering Strain (%)
En
gin
ee
rin
g S
tre
ss (
MP
a)
Load
Unload
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Conclusions
Choice of material model depends onmaterial
test data
situation complexity
Proper selection = reasonable model
Simple improvements can add power
Validated models represent baseline
Models can be tuned for multi-axial loadings
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